This invention relates to semiconductor devices and more particularly to semiconductor devices having an electronic component sealed within a package.
In the assembly of semiconductor devices, such as an integrated circuit (IC) device, or the like it is desirable to avoid exposing the devices to excessive heat that may cause damage to certain materials present in the device. Conventional materials and assembly processes used to form hermetic package seals often require high temperature process steps. For example, high temperature steps are often used to form a hermetic glass seal between the base and the lid of the package housing an electronic component. The glass sealing process is typically performed at temperatures ranging from about 400 to 500 degrees centigrade and heating times of about 1 to 2 hours. In this time and temperature range many glass materials begin to exhibit flow characteristics, which is necessary to form a glass seal, but the high temperature sealing process can also cause oxidation of many metals used to form package leads. For example, copper and aluminum must be plated with a non-readily oxidizable material such as nickel to prevent the formation of oxide scales on the lead surface. Additionally, the glass sealing process can cause cracking of the passivation layers overlying the circuit components of the IC device. Once the passivation layer has been compromised, the protection of the circuit elements against moisture and contamination has been lost and a potential for premature failure of the IC device exists.
The need to produce more reliable semiconductor devices has led to the development of assembly processes using low temperature sealing materials. Both thermoplastic materials and thermosetting polymers, such as epoxy resins, have been developed for use as sealant materials to enclose an IC device within a package body. Thermal epoxies that can be cured at temperatures between 150 and 200 degrees centigrade have been developed. The thermal epoxy curing process initiates cross-linking of low-molecular-weight polymer chains forming high molecular weight interlinked polymer units.
The use of epoxy sealant materials offers several advantages to semiconductor device manufactures and can be used for low temperature sealing of ceramic packages such as pin-grid-arrays (pgAs) and ceramic-quad-flat-packs (CQFPs). An low temperature epoxy sealant has potential to offer a highly reliable sealant in applications where a totally hermetic seal is not required. However, it has been observed that thermal epoxy materials can suffer defects during processing even at relatively moderate sealing temperatures.
In an assembly process using a thermal epoxy, a layer of epoxy is screened onto a ceramic base then partially cured using a heat cycle of short duration to partially bond the epoxy to the lid. This step is known in the art as B-stage processing. A ceramic lid is then clamped onto the base and the assembly is heated to drive of solvents present in the epoxy and to form polymer cross-links in the epoxy film. Typically, the base and the lid are configured such that when they are brought together a cavity is formed in the interior of the package. The cured epoxy forms a seal between the lid and the base enclosing the IC device within the cavity. This process has the advantage of producing a seal at a lower temperature than that used to form a glass seal; however, the process is sensitive to pressure differentials between the cavity and the ambient environment. During the curing process, air trapped inside the cavity can expand and escape the confines of the cavity through the curing epoxy film. The air traversing the thermal epoxy film creates holes or voids in the epoxy resulting in the formation of a structurally weak seal that is prone to rupturing. A similar problem can be encountered if the assembly is cooled too rapidly after curing the epoxy. With rapid cooling the air pressure inside the cavity can momentarily drop below that of the outside ambient and air can flow into the cavity through the warm epoxy film forming voids in the film. The benefit of using a thermal epoxy as a seal material can thus be compromised by a pressure differential forcing air through the epoxy as the film is cured.
The thermal epoxies overcome some of the problems of glass seals, but are themselves prone to seal integrity problems. Accordingly, a need existed for a semiconductor device having a package seal material capable of forming a seal at room temperature which overcomes the problems associated with high temperature glass seals and with epoxy seals, and a process for fabricating the same.